It has been discovered that airlaid webs comprising heat-sensitive binder material (thermoplastic or thermally curable) can be molded into useful, three-dimensional shapes providing improved body fit and flow control by means of an online process in which a flat airlaid web comprising binder material is held against a molding substrate after or during application of energy to the web, causing the web to conform to the molding substrate and form bonds that lock the web into the shape of the molding substrate. The web can be held against the molding substrate by pneumatic forces, tension in the web itself, tension applied by a belt or wire, restraining forces from a backing surface such as a second surface that conforms to the molding substrate, and the like. The molding substrate can be metal (e.g., aluminum, steel, copper, brass, titanium, and the like), glass, ceramic, plastic, a composite material, and the like, and can be gas permeable or impermeable.
The methods of the present invention can enable production at industrial speeds of molded absorbent structures shaped to provide both body fit and flow control in absorbent articles.
Molded airlaid structures within the scope of the present invention include those having slotted vertical gaps that offer an entranceway for gushes of body fluids, as well as anatomically conforming shapes adapted to not only receive and direct flowing fluids, but to also guide the flexure of the article in use to better conform to the body. Articles according to the latter concept can, for example, have a central elongated hump, one or more longitudinal flow channels, and a plurality of transverse flexure zones (hereafter described) away from the central hump to cause the article to flex toward the body when compressed from the sides. Such articles can adapt to fit the body of the wearer during the dynamic conditions of use, thereby providing both comfort and leakage prevention.
The energy can be applied by microwave radiation, radiofrequency energy, or other electromagnetic radiation sources, as well as by heated air or by conduction with heated surfaces. When microwave energy is applied, the web can incorporate binder materials such as thermoplastic binder fibers or curable resins that are relatively sensitive to microwave radiation (compared to pure cellulose itself) by virtue of a high dipole moment. In one embodiment, microwave energy is applied to the moving web as it passes through an opening in a microwave resonance chamber, where microwave energy is focused into the web. In another embodiment, microwave energy is applied to the web through a rotating microwave horn terminating in a microwave-transparent window having a three-dimensional structure suitable for molding the web. The rotating horn moves with the web while the web is near or in contact with the horn. A wide variety of other embodiments are also within the scope of the present invention, as set forth hereafter.
Alternatively, the energy can be applied in the form of heated gas passing through the web, or by conduction from one or more heated molding surfaces, or by application of ultrasonic energy, infrared energy, and the like. The energy heats the binder material, promoting fusion of a portion of the binder material to join fibers in the airlaid in the case of a thermoplastic binder, or promoting curing in the case of thermosetting materials or heat-curable crosslinking agents. The resulting molded airlaid web can be cut into discrete sections suitable for incorporation into an absorbent article. The section of airlaid web can have any one or more of the following characteristics: a substantially uniform density, an apparent thickness at least 50% greater than the original thickness of the unmolded web, an Overall Surface Depth (hereafter defined) of at least 0.5 mm, a Surface Height of at least 1 mm, a wet compressed bulk at least 50% greater than that of the unmolded web, a longitudinally elongated central hump and one or more transverse flexure zones between the hump and one or more of the longitudinal ends of the section of the airlaid web, and one or more longitudinal flow channels formed by elevated structures on the section of the airlaid web.
As used herein, an xe2x80x9cairlaid webxe2x80x9d is a fibrous structure formed primarily by a process involving deposition of air-entrained fibers onto a mat, typically with binder fibers present, and typically followed by densification and thermal bonding. In addition to traditional thermally bonded airlaid structures (those formed with non-tacky binder material present and substantial thermally bonded), the scope of the term xe2x80x9cairlaidxe2x80x9d according to the present invention can also include coform, which is produced by combining air-entrained dry, dispersed cellulosic fibers with meltblown synthetic polymer fibers while the polymer fibers are still tacky. Further, an airformed web to which binder material is subsequently added can be considered within the scope of the term xe2x80x9cairlaidxe2x80x9d according to the present invention. Binder can be added to an airformed web in liquid form (e.g., an aqueous solution or a melt) by spray nozzles, direction injection or impregnation, vacuum drawing, foam impregnation, and so forth. Solid binder particles can also be added by mechanical or pneumatic means.
As used herein, an xe2x80x9cairformed webxe2x80x9d refers to a mat comprising cellulosic fibers such as those from fluff pulp that have been separated, such as by a hammermilling process, and then deposited on a porous surface without a substantial quantity of binder fibers present. Airfelt materials used as the absorbent core in many diapers, for example, are a typical example of an airformed material.
In one embodiment, the absorbent article of the present invention has an upper absorbent layer comprising a three-dimensional molded cellulosic airlaid web having a portion of water-resistant thermoplastic binder material therein. The molded airlaid web can have a substantially uniform basis weight and thickness prior to molding, but is molded to have a plurality of elevated regions offering a distinctive profile well suited for conforming to the body of the wearer. The molded web can also be adapted for providing significant void volume beneath the upper absorbent layer and preventing leakage to the sides of the article. In some embodiments, the molded airlaid web has a body-side surface topography comprising a central hump having an oval shape elongated in the longitudinal direction, and a plurality of molded flexure zones having a component extending in the transverse direction and disposed between the central hump and at least one longitudinal end of the molded airlaid web. The molded flexure zones assist in permitting an initially flat article to readily conform to the shape of the wearer""s body along the longitudinal axis of the article.
In one embodiment, thermal molding is achieved as hot gas passes through the web in the region to be molded, causing the binder material to become activated (e.g., for thermoplastic material such as binder fibers to at least partially melt and bond cellulosic fibers together) to hold the web in the shape defined by the mold. Heat transfer may further be assisted by providing an oscillatory flow of heated gas with a reverse flow component, such as is found in the heated gases produced from pulsed combustion systems, wherein acoustic waves enhance the heat transfer of the gases. An exemplary system for providing oscillatory flow of heated gases suitable for the present invention is disclosed in U.S. Pat. No. 6,085,437, issued Jul. 1, 1998 to G. K. Stipp, herein incorporated by reference.
When shaping of the web comprises application of mechanical pressure from a solid surface, as opposed to pneumatic pressure, the web can be heated before the mechanical forces for shaping are fully applied in order reduce damage to the web and achieve higher strength and molding definition. Such preheating can be achieved with any known method, such as steam impregnation, heated air passing through the web, application of radiative or radiofrequency energy, and the like. Alternatively, the solid surfaces themselves may be heated to cause heating of the web sufficient to activate the binder material.
While webs can be heated by conduction, high-bulk cellulosic webs can be poor conductors and may not always permit uniform treatment of the web under the constraint of short heating times. Other forms of heat can be applied as the web is being held in a desired shape. Suitable forms include application of ultrasonic energy; radiofrequency energy such as microwaves, particularly when binder material in the airlaid web is responsive to such radiofrequency energy; and convective heating from hot gases passing through or impinging onto the web.
For many binder materials, heating to temperatures above about 90xc2x0 C. is required for effective activation of the binder material. For example, many thermoplastic binder materials become activated over a temperature range of about 95xc2x0 C. to 200xc2x0 C., more specifically from about 100xc2x0 C. to about 170xc2x0 C., and most specifically from about 110xc2x0 C. to 150xc2x0 C. The higher the temperature, the higher the molding definition. Excessive temperatures should be avoided to prevent scorching or other harm to the web.
The use of radiofrequency energy, microwaves or other electromagnetic means of applying energy to a web can allow more uniform treatment of the web or of any binder material in the web. As used herein, xe2x80x9cradiofrequencyxe2x80x9d (RF) energy comprises electromagnetic radiation in the spectral range of 300 Hz to 300 GHz. xe2x80x9cMicrowave radiationxe2x80x9d is a subset of RF radiation spanning the spectral range from 30 MHz to 300 GHz. Typical frequencies for microwave energy are 915 MHz and 2450 MHz (2.45 GHz), the ISM bands allowed by the Federal Communication Commission (FCC). General principles for microwave heating are given by R. C. Metaxas and R. J. Meredith in Industrial Microwave Heating, Peter Peregrinus, LTD, London, 1983. A useful tool in the design of microwave heating systems is the HFSS(trademark) software provided by Ansoft Corp. (Pittsburgh, Pa.).
In one embodiment, applying sufficient energy to the airlaid web comprises application of microwaves to cause components in the web to heat sufficiently to fuse or melt thermoplastic binder materials. For example, an airlaid web can comprise fibers and/or particles of dipolar polymers such as polyurethanes, isocyanates, polyethylene oxide, polyester, and their derivatives, or mixtures or copolymers formed therefrom. Application of microwave radiation causes the dipolar polymers to become heated enough to either fuse or to cause other less dipolar thermoplastic materials to fuse. For example, a sheath-core bicomponent fiber with a polyester core and a polyolefin sheath can be subjected to microwave radiation to cause the core to heat sufficiently to cause melting of the sheath without melting or degradation of the core. Alternatively, the sheath can be more microwave susceptible than the core. An exemplary application of microwave energy is found in the commonly owned PCT publication WO 99/22686, xe2x80x9cComposite Material with Elasticized Portions and a Method of Making the Same,xe2x80x9d by R. G. Brandon, F. M. Chen, and R. E. Vogt, U.S. Pat. No. 5,916,203, issued Jun. 29, 1999. Further details of providing microwave chambers for applying energy to a moving web are disclosed in U.S. Pat. No. 5,536,921, issued Jul. 16, 1996 to Hedrick et al.; U.S. Pat. No. 6020580; and U.S. Pat. No. 4,234,775, issued Nov. 18, 1980 to Wolfberg et al.; all of which are herein incorporated by reference.
U.S. Pat. No. 5,958,275, issued Sep. 28, 1999 to Joines et al., herein incorporated by reference, provides several useful embodiments for application of microwave energy to a moving planar material such as a web. The web passes through a slot in a microwave chamber that has adjustably variable path lengths to allow peaks and valleys of the electromagnetic field in one exposure segment to compensate for peaks and valleys in another exposure segment. For example, the microwave chamber may have a serpentine shape that makes several passes over the web to ensure uniform application of microwave energy. Specialized choke flanges prevent the escape of electromagnetic energy. One or more rollers between exposure segments in the microwave chamber may be enclosed by an outer surface to prevent the escape of electromagnetic energy.
In an embodiment related to the equipment disclosed in commonly owned U.S. application Ser. No. 09/603714 by R. E. Vogt, filed Jun. 27, 2000, herein incorporated by reference, microwave energy is directed by a waveguide into a resonance chamber adapted to focus energy into a plane or along a line through which the moving web passes. A cylindrical chamber can be suitable, for example, wherein the web travels along a diameter of the chamber, entering and leaving through slots along opposing sides of the cylinder. Quarter-wavelength chokes extend outward from the slots to prevent excess leakage of microwave radiation through the slots. When tuned for microwave energy to fill the chamber in the TM010 mode, the energy is focussed along the axis of the cylinder and thus into the web for efficient delivery of energy. (TM modes are generally expected to be useful for microwave heating of a web in the present invention. TEM modes can be used but are more likely to permit leakage of microwaves from the chamber.) The web may be carried on a belt of material such as Teflon(trademark) that is relatively unsusceptible to microwave energy, or the web can pass through the chamber without being on a carrier belt.
General principles for use of cylindrical resonance chambers for microwave heating and the coupling of a waveguide to an aperture in the cylinder are given by R. C. Metaxas and R. J. Meredith in Industrial Microwave Heating, Peter Peregrinus, LTD, London, 1983, pp. 183-195. In general, a rectangular waveguide is choked down through an aperture in the center of the cylinder (e.g., on the top or bottom of the central portion of the cylinder when a web runs through the middle of the cylinder along the horizontal diameter) to provide efficient transfer and distribution of microwave energy into the cylinder.
U.S. Pat. No. 6,020,580, issued Feb. 1, 2000 to Lewis et al., herein incorporated by reference, discloses a suitable microwave applicator with an elongated chamber such as a cylindrical shape which can be used or adapted in accordance with Vogt (U.S. application Ser. No. 09/603714) for use in the present invention. A waveguide, connected to the elongated chamber, couples microwave power into the elongated chamber. The cross-sectional area of the elongated chamber can be mechanically adjusted to control and maintain the microwave field uniformity and resonant mode, suitably a length independent mode TM010, during the processing of the material. The applicator thus provides microwave energy having a substantially uniform field distribution over a large area for processing a web.
In addition to heating and activating thermoplastic binder materials, electromagnetic radiation in the form of microwaves or ultraviolet radiation, for example, can also be used to cure resins that are in liquid form. For example, an airlaid web can be impregnated or sprayed with a liquid binder system, followed by application of light pressure to mold the web into a three-dimensional shape as radiation is applied to cure the liquid binder. Heat can also be applied to cure some binder systems, wherein the heat is applied by through drying or other convective means with hot gas passing into the web, infrared radiation, conduction, and the like. Examples of microwave and UV curing of resins in a fibrous preform is found in U.S. Pat. No. 5,169,571, issued Dec. 08, 1992 to D. T. Buckley, and in U.S. Pat. No. 5,338,169, issued Aug. 16, 1994, also to Buckley, both of which are herein incorporated by reference. One form of convective heat transfer of value in the present invention is the hot air knife, or HAK, as described in U.S. Pat. No. 5,962,112, issued Oct. 5, 1999 to Haynes et al., herein incorporated by reference.
A method for simultaneously applying microwave radiation and applying moderate pressure to the web to mold it can be achieved by using a microwave-transparent solid material, or microwave window, as one of the surfaces pressing against a bulky web. Suitable microwave windows and cooling systems for the windows are disclosed in U.S. Pat. No. 5,228,947, xe2x80x9cMicrowave Curing System,xe2x80x9d issued Jul. 20, 1993 to M. T. Churchland, herein incorporated by reference in its entirety.
While the above examples typically are directed toward a web passing through stationary microwave equipment, the microwave energy or other energy sources for heating the web can be mounted to a moving structure (or energy from a stationary source can be guided into and distributed from a rotating device), such as a rotating wheel, or an moving belt or track, to move with the web for a predetermined length or time. A plurality of energy sources can be provided on the moving structure. A portion of the web can then be treated by a moving energy source, and upon separation from the energy source, the energy source can be repositioned to treat another portion of the moving web. For example, 10 or more microwave sources can move on an endless track, permitting five or more to be acting portions of the web at any time. In this manner the web can be molded by a moving molding substrate at the same time energy is applied to it.
By way of example, a moving web can rotate on a turret with a plurality of microwave horns, each terminating in a microwave-transparent window that can be pressed against the web as microwave energy is applied. The horns can be supplied with microwave energy from one or more stationary sources via a waveguide leading into the center of the rotating turret, or one or more microwave sources can be installed inside a rotating turret. The three-dimensional shape of the web as it is pressed against the microwave window can be locked into place by the fusion of binder material that joins fibers together once the binder material cools again. Alternatively, the binder material can be thermosetting or curable, becoming solidified or activated upon heating to hold the fibers together in the three-dimensional shape experienced during application of energy. A microwave-reflecting backing surface can be present, with the web residing between the backing surface and the microwave horn, to prevent microwave leakage and help establish effective resonance for heating of the web or the binder material therein.
When using microwave energy supplied radially outward from a turret, the web can be held against the molding substrate with a belt that can be microwave transparent or microwave reflecting, such as a belt with a metallic mesh therein.
U.S. Pat. No. 6,001,300, issued Dec. 14, 1999 to Buckley, herein incorporated by reference, also discloses methods for applying microwave energy into a three-dimensional mold through a molding surface transparent to microwave energy. Waveguides are used to uniformly distribute the energy. The microwave windows can comprise a plurality of segments to reduce the risk of cracking from thermal stress. The windows also can be configured as lenses to direct the microwave energy to desired portions of the article being treated, following principles disclosed in Buckley. For the present invention, the microwave window may be flat, in cooperative relationship with an opposing molding substrate, or it may be three-dimensional, in cooperative relationship with an opposing three-dimensional surface or a flat surface. For example, the microwave window may be a male molding surface matched with an opposing female surface which act together to impose a three-dimensional pattern to a web as microwave energy is applied to fuse a microwave-susceptible binder material to cellulosic fibers of the web. The resulting molded web can have a substantially uniform density, or can be molded to have two or more zones of differing density in a pattern.
Prior to application of microwave energy, the web may be provided with a small amount of moisture, particularly water comprising ions, to increase the susceptibility of the web to microwave radiation and/or to increase the moldability of the cellulosic fibers. For example, a water spray adding 2 to 10 weight percent of water to the web can be effective in improving the energy absorption of microwaves and/or the conformability of the web. Water can be added by gravure printing, nebulizers, atomizers, fine water jets, or other techniques, either uniformly to the web or to discrete zones in the web where more molding or heating is needed. The microwave energy applied may then dry off any undesired water add-on, or further drying by through-drying or other means can be applied.
Two classes of binder materials can be considered: thermoplastic solid materials (particles or fibers), and liquids (e.g., resins or solutions) that can be cured or set by application of heat or other energy sources to provide dry, water-resistant bonds between fibers. The binder material can comprise about 50% or less of the dry mass of the cellulosic web, such as from about 5% to 45%, or from 5% to 25%, or from 6% to 15%.
For solid binder materials, any known thermoplastic material can be used as a binder, provided that the material can be fused at a temperature that does not destroy or render unsuitable the fibrous mat itself. A thermoplastic binder upon activation by heat becomes soft but reverts to its normal frozen state upon cooling. Representative of such thermoplastic binder materials are polypropylenes, polyethylenes, polycarbonates, polyvinyl chloride, polyesters, polystyrenes, acrylics and the like. The binder material may be hydrophobic or hydrophilic. Hydrophilic fibers can be inherently hydrophilic or can be a synthetic hydrophobic fiber that has been treated with a hydrophilic coating. Examples of hydrophilic binder fibers are given in U.S. Pat. No. 5,849,000, issued Dec. 15, 1998 to Anjur et al., herein incorporated by reference.
The binder material can be unicomponent fibers or bicomponent polymer fibers such as sheath/core fibers or side-by-side bicomponent fiber, having a first component with a lower melting point than the second component, such that upon heating to about the melting point of the first component, the first component can fuse and bond to nearby cellulosic fibers while the second component can maintain the integrity of the binder fiber. Examples include DANAKLON(copyright) bicomponent fibers of Hercules, Inc. (Wilmington, Del.); or PET (poly(ethylene terphthalate)) core fibers an activated co-polyethylene sheath, such as CELBOND(copyright) fibers produced by KoSA Inc. (formerly Trevira Inc. and formerly Hoechst-Celanese), Salisbury, N.C., under the designation T-255 and T-256. Other useful binder fibers include the copolyester fibers described by W. Haile et al. in the article, xe2x80x9cCopolyester Polymer for Binder Fibers,xe2x80x9d Nonwovens World, April-May 1999, pp. 120-124, or materials produced by ES FiberVisions Inc. (Wilmington, Del.). In addition to sheath/core fibers, components of a binder fiber having a plurality of polymers may be arranged in a side by side arrangement, a pie arrangement or an xe2x80x9cislands-in-the-seaxe2x80x9d arrangement, or in a blend. Conjugate fibers are taught in U.S. Pat. No. 5,108,820 to Kaneko et al., U.S. Pat. No. 5,336,552 to Strack et al., and U.S. Pat. No. 5,382,400 to Pike et al. For two component fibers, the polymers may be present in ratios of 75/25, 50/50, 25/75 or any other desired ratios. The fibers may also have shapes such as those described in U.S. Pat. No. 5,277,976 to Hogle et al., and U.S. Pat. Nos. 5,069,970 and 5,057,368 to Largman et al., hereby incorporated by reference in their entirety, which describe fibers with unconventional shapes.
Unicomponent fibers can include, by way of example, polyethylene microfibers marketed as PULPEX(trademark) fibers by Hercules, Inc. (Wilmington, Del.) or Eastman""s Kodel(copyright) 410 binder fiber. This fiber requires a minimum temperature of about 132xc2x0 C. for good bonding. CoPET B from Eastman Chemical Company is another commercially available binder material with an activation temperature of about 110xc2x0 C. or higher. (This material can also be used as a sheath. For example, a useful bicomponent fiber is a coextruded sheath/core bicomponent with 35% CoPET B and a 65% PET core.) The binder material can also be a microwave-sensitize material having a high dielectric loss constant (e.g., from about 1 to 1,000 measured at a frequency of 1 kHz) such that the binder material is heated more than the cellulosic fibers when microwave energy is applied. (Cellulose can have a loss factor on the order of about 0.06 at 1 kHz.) Exemplary materials include polyamide or polyvinyl methyl based hot melt adhesives and other thermoplastics known in the art. Polyether block amides, polyvinylchloride (PVC) and related compounds also have high loss factors. The material can have a loss factor much greater than that of cellulose.
Binder materials can also be applied as liquid resins, slurries, colloidal suspensions, or solutions that become rigid or crosslinked upon application of energy (e.g., microwave energy, heat, ultraviolet radiation, electron beam radiation, and the like). For example, Stypol XP44-AB12-51B of Freeman Chemical Corp., a diluted version of the Freeman 44-7010 binder, is a microwave-sensitive binder that was used by Buckley et al. in U.S. Pat. No. 6,001,300, issued Dec. 14, 1999, previously incorporated by reference. Buckley et al. also disclose the following UV-sensitive binders available from Freeman Chemical: 80497 (slow system), 747-10 (medium system) and 19-4837 (fast system).
Various types of thermosetting binders are known to the art such as polyvinyl acetate, vinyl acetate, ethylene-vinyl chloride, styrene butadiene, polyvinyl alcohol, polyethers, and the like, as well as elastomeric latex emulsions. Representative thermosetting binder materials which are adapted for application in the form of a liquid dispersion include copolymers of ethylene and acrylic acid, vinyl acetate-ethylene copolymers, acrylonitrile-butadiene copolymers, vinylchloride polymers, vinylidene chloride polymers, curable acrylic latex compositions, xe2x80x9cAirflexxe2x80x9d available from Air Products and Chemicals, P.O. Box 97, Calvert City, Ky. 42029, and the like.
Latex that does not become crosslinked can be useful in producing an absorbent article that is also flushable after use. For example, commercial latex sources can be used, wherein a crosslinker is present, without causing significant crosslinking if the temperature of curing is kept below a designated temperature (e.g., below 130xc2x0 C. for many latices), or if the pH is kept at a level incompatible with latex crosslinker (e.g., a pH of 8 or above, more specifically 8.5 to 10.8). Alternatively, a crosslinking inhibitor could be added to preclude crosslinking, even when heated. Sodium bicarbonate, for example, can be a useful crosslinking inhibitor. Also alternatively, latex can be prepared with substantially no crosslinker present (typically NMA), such that a water-dispersible film can form upon drying which can provide strength in the dry state and a reduced degree of strength when moistened, with the possibility of rapid break up when flushed.
Water-soluble, non-colloidal, cationic, thermosetting binders suitable for use with cellulosic fibers are disclosed in U.S. Pat. No. 4,617,124, issued Oct. 14, 1986 to Pall et al., herein incorporated by reference, where epoxide-based versions are said to be preferred, including both polyamido/polyaminoepichlorohydrin resins and polyamine-epichlorohydrin resins, such as Kymene(copyright) 557 and the Polycup(copyright) series of resins manufactured by Hercules Incorporated (Wilmington, Del.). Related materials can be prepared by reacting epichlorohydrin with condensation products of polyalkylene polyamides and ethylene dichloride. Compositions of this type are disclosed in U.S. Pat. No. 3,855,158 and are exemplified by Santo-res(copyright) 31, a product of Monsanto Inc. Another form of this particularly type of binder resin is prepared by the reaction of epichlorohydrin with polydiallyl methyl amine to produce an epoxide functional quaternary ammonium resin. Compositions of this kind are disclosed in U.S. Pat. No. 3,700,623 and are exemplified by Resin R4308, a product of Hercules Incorporated. The disclosures of U.S. Pat. Nos. 3,855,158 and 3,700,623 are incorporated herein by reference.
Water degradable binder fibers can be used such as those used in the coform products of U.S. Pat. No. 5,948,710, issued Sep. 7, 1999 to Pomplun et al., or those disclosed by Jackson et al. in U.S. Pat. No. 5,916,678, issued Jun. 29, 1999, both of which are herein incorporated by reference.
Polycarboxylic acids can also be used as thermally curable binder materials. For example, commonly owned U.S. patent application Ser. No. 09/426300, xe2x80x9cPatterned Application of Polymeric Reactive Compounds to Fibrous Webs,xe2x80x9d filed Oct. 25, 1999 by Sun and Lindsay, herein incorporated by reference in its entirety, discloses polymeric anionic reactive compounds which can be applied to cellulosic webs to cause crosslinking between the fibers for good strength and bonding. The polymeric reactive compound can be a polymer such as a copolymer, terpolymer, block copolymer, homopolymer, or the like, comprising a monomer with carboxylic acid groups on adjacent atoms (particularly adjacent carbon atoms) that can form cyclic anhydrides in the form of a 5-membered ring, with maleic acid or its derivatives representing specific embodiments of such a monomer. Copolymers of maleic acid or maleic anhydride are thus useful polymeric reactive compounds. Polyacrylic acid can be formed to be useful for the present invention if a significant portion of the polymer comprises monomer that are joined head to head rather than head to tail, to ensure that carboxylic acid groups are present on adjacent carbons. Copolymers of maleic acid or anhydride with acrylic acid or its derivatives are also useful polymeric reactive compounds. A useful commercial compound comprising polycarboxylic acids suitable for bonding fibers in an airlaid web is BELCLENE(copyright) DP80 from FMC Corporation, which is a terpolymer of maleic acid, vinyl acetate, and ethyl acetate.
Useful catalysts for curing with polycarboxylic acids include alkali metal salts of phosphorous containing acids such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates. Useful metal polyphosphonates can include sodium hexametaphosphate and alkali metal hypophosphites such as sodium hypophosphite. When a catalyst is used to promote bond formation, the catalyst is typically present in an amount in the range from about 5 to about 20 weight percent of the polycarboxylic acid. More specifically, the catalyst can be present in an amount of about 10 percent by weight of the polycarboxylic acid. A variety of suitable catalysts are described in U.S. Pat. No. 4,820,307, issued Apr. 11, 1989 to Welch et al., herein incorporated by reference. Other useful catalysts include sodium phosphate, sodium sulfate, imidazole, carbodiimide, triethyl amine, and salts of unsaturated dicarboxylic acids.
Oven-curing of cellulose fabrics with polycarboxylic crosslinkers is disclosed by Kitchens et al. in U.S. Pat. No. 5,042,986, issued Aug. 27, 1991, herein incorporated by reference. Curing is performed at about 150-240 degrees Celsius for 5 seconds to 30 minutes, with the lowest time reported as actually used being 15 seconds. Still faster methods (flash curing) are disclosed in commonly owned, copending U.S. application Ser. No. 09/425810, xe2x80x9cFlash Curing of Fibrous Webs Treated with Polymeric Reactive Compounds,xe2x80x9d filed Oct. 25, 1999 by Sun and Lindsay, herein incorporated by reference.
Binders applied in liquid or solution form to the fibrous web can include any of the binders described in U.S. Pat. No. 5,609,727, issued Mar. 11, 1997 to Hansen et al., herein incorporated by reference.
The binder material can be selected for cost and performance attributes. The binder may optionally contain various fillers, pigments, dyes, etc. if desired.
Binder materials can also be biodegradable and can include polylactic acid and biodegradable polyesters.
The molded airlaid webs used in the present invention can have non-planar surfaces on both sides of the web, in contrast to many previous attempts at providing contours in absorbent articles by nonuniform distribution of mass, wherein one side of the contoured absorbent layer is typically flat. The three-dimensional structure of the entire web, not just a single surface, in some embodiments can provide flow channels and void spaces on both sides of each molded airlaid web to help guide fluid flow and provide additional absorbent capacity.
The density of the web need not be substantially uniform, and can have density gradients to provide capillary pressure gradients for fluid transport. For example, outer portions of a web can have a higher density, or a lower layer of the web can have a higher density to preferentially wick fluid toward the high density zone. The web can be heterogeneous in composition, such as having a portion of polyolefin fibers in the lower layer and substantially all cellulosic fibers in the upper lay. Such webs can be made by introducing various fibers into the airlaying process at different positions or times during formation of the web, or by joining a plurality of layers to form one integral layer. In some embodiments, the upper layers of a fibrous web can be more hydrophobic than the lower layers to create a dry feel on the skin.
When more than one molded airlaid web layer is used, the topography of each molded airlaid web can be the same or similar to other molded airlaid webs in the core, or one layer can differ from another. For example, large sinusoidal peaks (e.g., base width greater than 4 mm, height about 2 mm or greater, spaced apart in a grid) can be formed in one airlaid web, while the other layer is molded with a sine wave having a different frequency or can have a different pattern altogether to prevent nesting and increase the void volume between adjacent layers. The interaction of the molded areas in two or more of the airlaid webs results in a central hump. The material properties of the layers can give the hump resiliency, such that it can be depressed with a cushiony feel but pops up when released, even when wet, in part by virtue of the bonds in an airlaid web formed from heating or curing of the binder material while the airlaid web is in a three-dimensional state.
The body-side surface of the hump can serve as an intake region. In one embodiment, the hump is at least partially isolated from surrounding portions of the absorbent core by means of a wicking barrier to promote a center fill effect in fluid intake and to prevent fluid from traveling laterally from the hump to the longitudinal sides of the article. Thus, the absorbent core can comprise an outer absorbent member having a central void or depression therein for receiving a central absorbent member comprising the hump formed by multiple layers of molded airlaid webs, with a wicking barrier such as a polymeric film lining the central void to prevent or hinder fluid communication between the central absorbent member and the outer absorbent member. The wicking barrier in this embodiment can provide not only a flow barrier directly between the central absorbent member and the outer absorbent member spanning a vertical distance, but also extends outward from the central absorbent member on the body-side surface of the outer absorbent member to define a ledge or horizontal component of the wicking barrier which can help prevent fluid communication between the central absorbent member and the outer absorbent member when the absorbent core is laterally compressed and bunched together. Principles for product design and construction employing a wicking barrier in the absorbent core are disclosed in commonly owned, copending U.S. patent application Ser. No. 09/165875, xe2x80x9cAbsorbent Article Having Integral Wicking Barriers,xe2x80x9d by Chen et al., filed Oct. 2, 1998, herein incorporated by reference.
When a wicking barrier is used in combination with a structure having a central absorbent member and outer absorbent member, the central portion of the absorbent core need not be completely separate from the surrounding outer portions of the absorbent core. Each layer can be a unitary absorbent layer having a contoured central portion which contributes to a hump in the absorbent core, wherein a wicking barrier longitudinally separates the hump from the longitudinal sides of the article without completely isolating the central portion. The use of wicking barriers to separate an outer portion of a unitary absorbent layer from a central portion thereof in the crotch region of the absorbent article is described in more detail in commonly owned copending application xe2x80x9cAbsorbent Article with Unitary Absorbent Layer for Center Fill Performance,xe2x80x9d Ser. No. 09/411261 by J. D. Lindsay et al., filed Oct. 1, 1999, herein incorporated by reference. A unitary absorbent layer has an outer portion and an inner portion with a wicking barrier forming part of the boundary therebetween, but the inner and outer portions are still contiguous rather than separate members.
Any known topsheet material can be used in the absorbent articles of the present invention. While the topsheet can be added to the molded airlaid web after molding has been completed, especially good visual definition of the contoured surface can be achieved in some embodiments when the topsheet is disposed over the airlaid web prior to molding. Thus, in one embodiment, a topsheet that can comprise thermoplastic fibers such as polyolefin materials, is disposed over an airlaid web, followed by a molding step to permanently mold the airlaid web to have a three-dimensional topography. The molding step can comprise deforming the airlaid web and topsheet between two opposing molded surfaces (e.g., male and female patterns in cooperative association) as energy is applied to the airlaid web to cause bonding of thermoplastic binder material therein or activation or heat-setting resins. When high temperatures are desired for molding of a cellulosic web, such as above 160xc2x0 C., above 180xc2x0 C., above 200xc2x0 C., or above 230xc2x0 C., high temperature polymers can be used in the topsheet to prevent undesired melting. Representative high-temperature polymers include polyesters such as polyethylene terephthalate or polypropylene terephthalate, polyamide fibers such as nylon 66 or MII fibers (Material Innovation, Inc., Leonia, N.J.), aramid fibers such as Kevlar(trademark), and the like.
The topsheet can also be apertured, or coapertured with the airlaid web, and may be further provided with slits, such as longitudinal slits along the sides of the airlaid web in the absorbent article.
In another embodiment, the airlaid web comprises elevated xe2x80x9cclamshellxe2x80x9d structures analogous to the distinctive sectional shapes of the famous Sydney Opera House, wherein raised arcuate projections terminate abruptly with cliff-like precipices (xe2x80x9cslotted gapsxe2x80x9d ) that can have open apertures spanning a vertical distance. In this embodiment, an elevated portion of the airlaid web is associated with a slit or other break in the airlaid web to allow the elevated portion to form a vertical gap defined by differing elevations on the respective sides of the slit or break (vertical relative to the plane of the article, assumed to be held in a horizontal position). With the vertical gap facing toward the center of the absorbent article, fluid can be intercepted and trapped that might otherwise run off the article, while the elevated arcuate structures can provide improved body fit and serve as barriers or dams to hinder liquid flow. In forming such xe2x80x9cclamshellxe2x80x9d structures, the airlaid web can first be slit and then molded to form the elevated portions with vertical gaps opening to void spaces beneath the elevated portions.
Possible uses of the present invention include absorbent articles for intake, distribution, and retention of human body fluids. Examples include feminine care pads and related catamenial devices or sanitary napkins, including xe2x80x9cultra-thinxe2x80x9d pads and pantiliners and maxipads. Examples of ultra-thin sanitary napkins are disclosed in U.S. Pat. Nos. 4,950,264 and 5,009,653 issued to Osbom; and U.S. Pat. No. 5,649,916, issued Jul. 22, 1997 to DiPalma et al., each of which are herein incorporated by reference in their entireties. Likewise, the present invention can be applied to diapers, disposable training pants, disposable incontinence pants or pull-ups, menstrual pants, other disposable garments such as incontinence pads, bed pads, sweat absorbing pads, shoe pads, bandages, and the like. The present invention can also be incorporated in articles adapted for particular portions of garments to be worn on the human body, gaskets for ostomy bags, and medical absorbents and wound dressings. The articles of the present invention can provide significant leakage protection, fluid center-fill absorptive performance, and other desirable attributes for absorbent articles.
For feminine care pads in particular, the present invention offers surprising advantages in terms of comfort and fit. The combination of two or more layers of molded airlaid webs having overlapping or superposed elevated regions generally yields a sharply defined hump in the absorbent core that appears to have been formed by molding an upper layer around a central pledget or other insert, when in fact no additional material is needed in the absorbent core. Further, the hump has a cushiony, resilient feel, being able to spring back after depression, even when wet, but being more comfortable and compliant than a hump created by insertion of a central pledget beneath an airlaid layer. Further still, the hump has substantial void space beneath it and can have substantial void space between the layers of airlaid web that make up the hump, depending on the topography of each layers and the propensity for the layers to nest together. Non-nesting patterns can be used in some embodiments to increase the void space within or beneath the hump, and to improve the resilience of the hump.
The molded airlaid webs of the present invention can provide useful and novel intake materials for acquisition of large volumes of urine in diapers and related articles while providing improved body fit to prevent leakage. Further, embodiments can also be made adapted to receive runny bowel movement in diapers, holding the fecal matter in void spaces beneath elevated fluid traps accessible through vertical gaps in the topography of the molded airlaid web.
The molded airlaid webs of the present invention are not restricted to structures with a central elevated hump, but can have a somewhat inverted form with a central depression that serves to receive body fluids such as runny bowel movement or urine. The molded web can be placed in an absorbent article with a larger section of absorbent material to interact with other members such as elastic gathers (e.g., elastic gathers disposed in the main absorbent core around the longitudinal edges of the molded web) to form a cup-like shape in use that fits about the crotch region of the user, particularly for male incontinence briefs. In one embodiment for use in male incontinence devices, the molded web has a central molded dome that is concave toward the body rather than the convex toward the body configuration that is often preferred for sanitary napkins. Methods of adapting a rectangular pledget through interaction with elastic gathers to form a cup-like shape in the crotch region are disclosed in U.S. Pat. No. 4,904,249, xe2x80x9cAbsorbent Undergarment with Fluid Transfer Layer and Elasticized Crotch Design,xe2x80x9d issued Feb. 27, 1990 to Miller et al., herein incorporated by reference. Replacing the pledget with a molded airlaid web can achieve a useful effect in such articles.
As used herein, the term xe2x80x9cactivatexe2x80x9d when used in reference to a binder material in a fibrous web receiving energy from an energy source means to convert the binder material to a state wherein improved bonding of fibers is possible. The binder material can be said to be activated when, for a thermoplastic material, at least part of the binder material becomes viscous upon application of the energy and flows to connect fibers together after it is resolidified,. If the binder is initially a liquid, dispersion, slurry, or other liquid-like material, the binder material is activated when it becomes relatively rigid (e.g., crosslinked or cured) or substantially solid. Thus, both thermosetting resins and thermoplastic materials can be activated by application of heat, though the cellulosic web is not fully set in the case of thermoplastic binder materials until the viscous heated thermoplastic material has resolidified after heat application has ceased.
As used herein, xe2x80x9cbulkxe2x80x9d and xe2x80x9cdensity,xe2x80x9d unless otherwise specified, are based on an oven-dry mass of a sample and a thickness measurement made at a load of 0.34 kPa (0.05 psi) with a 7.62-cm (three-inch) diameter circular platen. Thickness measurements of samples are made in a TAPPI-conditioned room (50% relative humidity and 23xc2x0 C.) after conditioning for at least four hours. Samples should be essentially flat and uniform under the area of the contacting platen. Bulk is expressed as volume per mass of fiber in cc/g and density is the inverse, g/cc.
Other measures relating to the height, elevation, or thickness of the molded airlaid web and its elevated structures are defined hereafter in connection with FIG. 1.
As used herein, the term xe2x80x9ccellulosicxe2x80x9d is meant to include any material having cellulose as a major constituent, and specifically comprising at least 50 percent by weight cellulose or a cellulose derivative. Thus, the term includes cotton, typical wood pulps, nonwoody cellulosic fibers, cellulose acetate, cellulose triacetate, rayon, thermomechanical wood pulp, chemical wood pulp, debonded chemical wood pulp, milkweed, or bacterial cellulose.
As used herein, the xe2x80x9ccrotch regionxe2x80x9d of an absorbent article refers to that region of the article in closest proximity to the crotch of the user, near the lowermost part of the torso, and resides between the front and rear portions of the article. Typically the crotch region contains the transverse centerline of the article and generally spans approximately 7 to 10 cm in the longitudinal direction.
Many articles of the present invention are intended to be worn near the crotch of a wearer, and thus have crotch regions. However, the present invention can also be applied to other articles such as underarm pads or wound dressings where a crotch region may not exist. In such cases, the article will have a region where fluid intake is intended to occur, termed the xe2x80x9ctarget region.xe2x80x9d The portion of the article including the longitudinal length of the target region and the full transverse width of the article normal to length of the target region is defined herein as the xe2x80x9ctarget zone.xe2x80x9d For articles intended to be worn in the crotch, the terms xe2x80x9ctarget zonexe2x80x9d and xe2x80x9ccrotch regionxe2x80x9d are generally synonymous, hereas xe2x80x9ctarget regionxe2x80x9d generally excludes the portions of the absorbent core near the ongitudinal sides since the intended area for fluid intake is generally substantially central in the absorbent article.
As used herein, the term xe2x80x9cextensiblexe2x80x9d refers to articles that can increase in at least one of their dimensions in the x-y plane by at least 10% and optionally at least 20%. The x-y plane is a plane generally parallel to the faces of the article. Principles for production of an extensible article are disclosed in U.S. Pat. No. 5,611,790, issued Mar. 18, 1997 to Osborn, herein incorporated by reference. In the case of a sanitary napkin comprising an absorbent core, for example, the article and the absorbent core can be extensible both in length and width. The absorbent article, however, may only be extensible in one of these directions. The article can be extensible at least in the longitudinal direction.
As used herein, the term xe2x80x9cflexure-resistantxe2x80x9d refers to an element which will support a bending moment, in contrast to an element which will support only axial forces. Likewise, as used herein, xe2x80x9cflexure resistancexe2x80x9d is a means of expressing the flexibility of a material or article and is measured according to the Circular Bend Procedure described in detail in U.S. Pat. No. 5,624,423, issued Apr. 29, 1997 to Anjur et al., herein incorporated by reference in its entirety. Flexure resistance is actually a measurement of peak bending stiffness modeled after the ASTM D4032-82 Circular Bend Procedure. The Circular Bend Procedure of Anjur et al. is a simultaneous multidirectional deformation of a material in which one face of a specimen becomes concave and the other face becomes convex. The Circular Bend Procedure gives a force value related to flexure-resistance, simultaneously averaging stiffness in all directions. For comfort, the absorbent article can have a flexure-resistance of less than or equal to about 1,500 grams, more specifically about 1000 grams or less, more specifically still about 700 grams or less and most specifically about 600 grams or less. For shaping performance, the central absorbent member as well as the outer absorbent member can have a flexure resistance of at least about 30 grams, more specifically at least about 50 grams, and most specifically at least about 150 grams.
As used herein, the term xe2x80x9cflexure zonexe2x80x9d refers to a region of an airlaid web that can readily bend due to the shape of the web to permit the web to conform to the body of the wearer. For good body fit along the longitudinal axis, flexure zones extending in the transverse direction are desired. A flexure zone is typically defined by the molded geometry of the molded airlaid web and can be a thin, elongated region of material corresponding to a cusp between two curves regions or to a region where a sudden change in material properties or material curvature occurs. For example, in a longitudinal cross-sectional profile having the shape of two concave down semicircles joined at the ends (similar to the digit xe2x80x9c3xe2x80x9d rotated to the left by 90 degrees), the middle portion of the material where the two semicircles join has a cusp-like quality. While a cusp in mathematics has no physical dimensions, the cusplike region between adjacent elevated structures can be a region of finite width (distance between the ends of the adjoining semicircles, for example), such as a relatively flat band between domelike elevated regions having a finite length of about 5 mm or less, more specifically about 2 mm or less, more specifically still from about 0.2 mm to about 2 mm, and most specifically from about 0.3 mm to about 1 mm. If the portion of the longitudinal profile shape with a cusplike region extends substantially in the transverse direction (as if the profile shape were extruded into the transverse direction), the airlaid web may fold along the longitudinal axis about a line or band comprising the cusplike region and serving as the flexure zone. The flexure zone can also be a region that has been densified by embossing or thermal or ultrasonic bonding, or that has been weakened by slitting or creased by folding, such that the interaction of the mechanical properties of the surrounding regions with the mechanical properties of the flexure zone promotes bending of the article about the flexure zone in use.
As used herein, the term xe2x80x9chorizontal,xe2x80x9d refers to directions in the plane of the article that are substantially parallel to the body-side surface of the article, or, equivalently, substantially normal to the vertical direction of the article (when the article can be held lying in a horizontal plane), and comprises the transverse direction and the longitudinal direction of the article, as well as intermediate directions. The orientation of components in an article, unless otherwise specified, is determined as the article lies substantially flat on a horizontal surface.
As used herein, the term xe2x80x9chydrophobicxe2x80x9d refers to a material having a contact angle of water in air of at least 90 degrees. In contrast, as used herein, the term xe2x80x9chydrophilicxe2x80x9d refers to a material having a contact angle of water in air of less than 90 degrees.
As used herein, xe2x80x9cOverall Surface Depthxe2x80x9d is a topographical measurement of the elevation difference that occurs on the surface of a three-dimensional web. Principles for the measurement and suitable equipment are described by Chen et al. in U.S. Pat. No. 5,990,377, xe2x80x9cDual-Zoned Absorbent Webs,xe2x80x9d issued Nov. 23, 1999, herein incorporated by reference The measurement is made by examining height data from moirxc3xa9 interferometry for a molded airlaid web resting on a horizontal surface with the body-side surface facing upwards. A profile line that encompasses the extremes of height (maximum and minimum) found on the upper surface (body-side surface) of the molded airlaid web, excluding apertures, is taken and analyzed. The difference in elevation between the 90% material line (a line at an elevation such that 90% of the length of the line along the profile is beneath the surface of the sample) and the 10% material line (a line at an elevation such that 10% of the length of the line along the profile is above the surface of the sample) in the two-dimensional profile comprising the extremes in height is taken as the Overall Surface Depth. For Overall Surface Depths greater than about 1.5 mm, the commercial moire interferometer described in U.S. Pat. No. 5,990,377 may require combination of data from two or more scans made with different focal planes to obtain data over a larger height range than is possible with a single measurement, or a moire interferometer with a larger vertical span can be adapted for use. The Overall Surface Depth can be about 0.5 mm or greater, more specifically about 1 mm or greater, more specifically still about 3 mm or greater, more specifically still about 6 mm or greater, and most specifically about 12 mm or greater, with exemplary ranges of from 4 mm to 10 mm, or from 5 mm to 15 mm, or from 2 mm to 25 mm. The Overall Surface Depth value is related to the simpler Surface Height measurement, hereafter described, but typically the Overall Surface Depth has a somewhat lower numerical value.
As used herein, a xe2x80x9cpledgetxe2x80x9d refers to an absorbent insert within an absorbent core having at least one of a width and a length smaller than the respective width and length of the absorbent core. A pledget is generally used to cause deformation or shaping of an adjoining layer of an absorbent article, and in the present invention, can be of use in shaping a pad or creating a medial hump in the pad for improved fit against the body of the wearer.
The term xe2x80x9csanitary napkinxe2x80x9d, as used herein, refers to an article which is worn by females adjacent to the pudendal region that is intended to absorb and contain the various exudates which are discharged from the body (e.g., blood, menses, and urine). While the present invention is shown and described in the form of a sanitary napkin, it should be understood that the present invention is also applicable to other feminine hygiene or catamenial pads such as panty liners, or other absorbent articles such as diapers or incontinence pads. The term xe2x80x9cfeminine care padxe2x80x9d as used herein is synonymous with sanitary napkin.
As used herein, xe2x80x9cSurface Heightxe2x80x9d is difference between the elevation of the highest region of a molded airlaid web as it rests on a flat, horizontal surface (elevation being taken relative to the flat surface) with the body-side surface facing upward and the local thickness of the molded airlaid web at prior to molding. If the web prior to molding did not have a substantially uniform thickness, the local thickness to be subtracted from the maximum elevation is taken at the region where the maximum will occur after molding. Elevation can be measured with any suitable method, including moire interferometry or use of a contact stylus applying a force small enough to not cause any noticeable deformation of the molded airlaid web.
As used herein, the term xe2x80x9ctransversexe2x80x9d refers to a line, axis, or direction which lies within the plane of the absorbent article and is generally perpendicular to the longitudinal direction. The z-direction is generally orthogonal to both the longitudinal and transverse) centerlines. The term xe2x80x9clateralxe2x80x9d refers to substantially in-plane directions having a predominately transverse component. Likewise, xe2x80x9cinwardly lateral compressionxe2x80x9d refers to compression directed from the longitudinal sides of an article toward the longitudinal centerline thereof, applied substantially in the transverse direction.